Gear pumps used for pumping hydraulic fluid utilize a drive gear and an idler gear which mesh proximate inlet and discharge openings of the pump. As the drive gear rotates the idler gear, there is, for each tooth, always a point a contact between the leading flank of the drive gear and trailing flank of the idler gear. Since the point of contact changes for each engagement between a drive tooth and idler tooth, there are really points of contact and, since the teeth have width, the points of contact are actually lines of contact. There is a clearance between the trailing tooth flanks of the drive gear and leading tooth flanks of the idler gear, which clearance is known as "backlash."
As the drive gear and idler gear rotate, hydraulic fluid fills the gap between adjacent teeth and is carried from the inlet, through an adjacent transition zone to the outlet. The fluid adjacent the outlet is prevented from crossing into the meshing area and is forced from the gap between mating teeth and pumped out of the outlet to a discharge line, where it is used to power components in an associated hydraulic circuit. When there is sealing action in the mesh of the teeth and displacement of fluid by a mating tooth, the pump is classified as a "positive displacement gear pump."
Having a relatively large displacement for a given center distance, positive displacement gear pumps experience difficulty filling the spaces between teeth when operated at high speeds. Inadequate filling causes dissolved air to come out of solution which results in entrained air bubbles. Since hydraulic fluid contains approximately 8% dissolved air by weight, the number of air bubbles can be considerable. As the hydraulic fluid is rapidly pressurized in the pump, the bubbles collapse, which results in cavitation damage to the pump in the pressure transition region between inlet pressure and outlet pressure. As the speed of the pump increases and/or as discharge pressure increases, pump damage occurs more quickly and is more intense.
Higher pump speed increases the probability that air will out-gas from the hydraulic fluid while higher pump speed decreases the time spent in the transition zone. In addition, the higher the pressure, the faster the entrained air bubbles collapse. This phenomenon, known as asymmetrical bubble collapse, results in fluid jets of extremely high velocity which cause localized pressure spikes as high as 100,000 psi. When these jets impinge upon surfaces within the gear pump they cause deep pitting over time which damages the pump.
Considering this matter further, bubble formation and subsequent collapse occurs in the region near the mesh as the volume rapidly increases. This is because fluid cannot fill the increasing volume at a sufficient rate which causes a very rapid pressure drop. This creates a vacuum which causes some of the air dissolved in the liquid to come out of solution forming the bubbles. As the bubbles are carried into the inlet cavity, the air pressure increases and the bubbles begin to dissolve back into the fluid. With hydraulic pumps running, at speeds of around 2,150 rpm, there is a rotational velocity of 13.degree. per millisecond. Assuming 120.degree. of rotation, the elapsed time from bubble formation in the mesh region to collapse in the high pressure region is about 9 milliseconds. If the air cannot redissolve in 9 milliseconds, cavitation damage occurs.
Cavitation damage to gear pumps is a problem not only with hydraulic gear pumps, but with other high speed gear pumps as well, such as gear pumps used to pump fuel for aircraft engines.